This proposal addresses the mechanistic and genetic basis for the Mre11 complex's role in the suppression of malignancy. Using yeast and mouse models, it focuses primarily on DNA repair and DNA damage signaling alterations imparted by alleles of RAD50. We have established that extreme cancer predisposition results from mutant combinations of RadSOS allele in the mouse, and determined the target cell of this pathology to be the hematopoietic stem cell. Further supporting the Mre11 complex's critical role in the suppression of malignancy is the fact that two distinct human chromosome instability syndromes associated with cancer predisposition are caused by mutations affecting the complex. Understanding its mechanism of tumor suppression is thus a high priority. The governing hypothesis of this proposal is that the Mre11 complex suppresses tumorigenesis through its affect on chromosome integrity in addition to its influence on DNA damage signaling. We address the cancer preventing functions of the complex in three primary foci. First we have established reagents and experimental strategies to define the structural role of the Mre11 complex in recombinational DNA repair in three contexts: during normal proliferation, in response to clastogens in vegetatively growing cells, and in meiotic recombination. Second, we will address the idea that the nuclease function of the Mre11 complex, and its contribution to the resolution of covalent protein-DNA complexes influences the suppression of malignancy by these highly conserved proteins. Third, the role of the Mre11 complex in post-mitotic cells will be examined.